The objectives of this study are two fold: 1) localize human placental relaxin at the subcellular level and; 2) develop a homologous human relaxin radioimmunoassay. Both problems should provide additional information for the role of the hormone in human parturition. The approach to these problems include: 1) Purification of relaxin from human placentas of normal term deliveries and Cesarean sections by gel filtration, ion exchange and isoelectrofocusing. 2) The purified human relaxin will be used to develop a specific antibody and a homologous radioimmunoassay. Competitive binding studies with FSH, LH, human chorionic gonadotrophin, insulin and proinsulin will be used to assess the specificity of the antiserum. 3) Fresh placental tissue from normal term deliveries, elective Cesarean sections and early stages of pregnancy will be processed for light and electron microscopy. The peroxidase-antiperoxidase technique will be used to identify the subcellular source of the hormone. The human relaxin antiserum will be utilized in these studies. The development of a homologous radioimmunoassay will provide a sensitive assay for the study of relaxin levels during pregnancy while the localization study should provide information for the packaging and release of relaxin from the human placenta.